Corneal permeability in a redesigned corneal holder for the bovine cornea opacity and permeability assay

The bovine cornea opacity and permeability assay (BCOP) is a proposed alternative to the Draize rabbit test for potential eye irritants. In the standard BCOP, bovine corneas are mounted in a holder on a flat surface between two identical chambers. The flat configuration of the standard holder does not conform to the normal curved shape of the bovine cornea and it comes into direct contact with the cornea tissue. Mounting corneas in this holder causes extensive damage to both epithelial and endothelial corneal cell layers. Our laboratory has designed a new holder that allows the cornea to maintain its natural curvature and does not damage the cornea. Previous tests, using both the new and standard holders, and comparing corneal opacity, hydration and endothelial morphology, have shown that the new holder is a significant improvement over the standard holder. The present study extends the comparisons of the new and standard holders to measurement of corneal fluorescein permeability. The permeability (ng/cm(2)/min) of intact corneas, corneas with no epithelium, and corneas treated with 1% NaOH, isopropanol, acetone, 30% trichloroacetic acid or 30% sodium dodecysulfate for either 1 or 10 min was determined by measuring fluorescence of samples taken from the endothelial chamber after 90 min epithelial exposure to 0.04% sodium fluorescein. In all trials, the redesigned holders yielded not only lower permeability measurements but also decreased measurement variability. The data provide further evidence that the new holder is an improvement over the standard holder and should be incorporated into a new protocol for the BCOP.

A redesigned corneal holder for the bovine cornea opacity and permeability assay that maintains normal corneal morphology

The bovine cornea opacity and permeability assay (BCOP) has been in use for nearly 10 years but has not been submitted for regulatory approval. In previous reports we have presented corneal hydration and endothelial damage as additional endpoints in this assay and have suggested that the design of the BCOP's corneal holder should be modified. The standard holder used in the BCOP assay induces physical damage to the cornea because it contacts clear cornea causing edge damage to the epithelial, stromal and endothelial layers. Second, by forcing a curved, oval-shaped bovine cornea into a flat, circular opening, corneal wrinkling occurs which can alter the cornea's optical characteristics and, most importantly, induces endothelial damage. We now report on a redesigned BCOP corneal holder that clamps onto the sclera, maintains normal corneal shape and does not cause damage to the endothelium. This ensures that irritancy tests are conducted using healthy, anatomically normal tissue. Tests of this holder using acetone, trichloroacetic acid, isopropanol and benzalkonium chloride show that it is now possible to evaluate effects of chemical substances on the endothelium. The effects of these compounds on corneal opacity and hydration in the new holder are similar to their effects on the cornea in the standard holder.

The eicosanoid, 15-(S)-HETE, stimulates secretion of mucin-like glycoprotein by the corneal epithelium

PURPOSE

The eicosanoid, 15-(S)-hydroxyeicosa-5Z, 8Z-11Z, 13E-tetraenoic acid (15-(S)-HETE), is known to stimulate production of mucin glycoprotein by airway epithelium. This study investigated the effect of 15-(S)-HETE on the mucin glycoprotein secretion by the corneal epithelium.

METHODS

To determine the effect of dose, corneas of anesthetized New Zealand White rabbits were treated with 50, 500, or 5,000 nM 15-(S)-HETE in artificial tears for 120 minutes. To determine the time to onset of the response, corneas were treated with 500 or 1,000 nM 15-(S)-HETE in balanced salt solution for periods ranging from 5 to 120 minutes. Corneas were fixed for electron microscopy in fixative containing 0.5% cetylpyridinium chloride (CPC) to stabilize the layer of mucin-like glycoprotein on the corneal surface. The mucin layer thickness was measured by image analysis of electron micrographs.

RESULTS

The layer of CPC-fixed mucin-like glycoprotein on the surface of control corneas was 0.46 +/- 0.04 microm thick. After treatment with 15-(S)-HETE, the thickness of the mucin layer increased to 0.64 +/- 0.1 microm at 50 or 5,000 nM HETE and as much as 1.02 +/- 0.2 microm in response to 500 nM HETE. Mucin thickness reached a statistical maximum of 0.59 +/- 0.1 microm after only 5 minutes of exposure to 500 or 1,000 nM HETE.

CONCLUSIONS

Exposure of the cornea to 15-(S)-HETE causes a rapid-onset increase in the thickness of a layer of mucin-like glycoprotein on the surface of the corneal epithelium. This supports previous reports that corneal epithelial cells produce mucin and suggests that treatment with topical 15-(S)-HETE may be effective in treating ocular surface mucin deficiency in dry eye syndrome.

Corneal opacity, hydration and endothelial morphology in the bovine cornea opacity and permeability assay using reduced treatment times

The purpose of this study was to determine whether the standard bovine cornea opacity and permeability (BCOP) assay exposure time of 10 minutes overestimates the ocular irritancy of chemical substances. Corneas were subjected to BCOP protocol following 30-second and 1-minute exposures to irritants. Corneal opacity and hydration (mg H(2)O/mg cornea) were then measured and compared to data obtained after 10 minute irritant treatments. For most test substances corneal opacity and hydration were lower following reduced exposure times. It is suggested that using shorter exposure times in BCOP protocol may be more predictive of human response to ocular irritants, since irritants are usually in brief contact with the ocular surface during accidental exposure. A second purpose of this study was to examine effects of irritants on the corneal endothelium. Corneas were treated according to BCOP protocol following exposure to irritants for 1 or 10 minutes. The endothelium was stained with Alizarin Red and trypan blue, and examined using light microscopy. Severe irritants, such as NaOH and trichloroacetic acid, cause endothelial cell death. It was also determined that simply mounting the cornea in the BCOP assay holders caused damage to 20% of the endothelial cells. Because the endothelium is essential for normal corneal transparency and hydration, it is suggested that examination of the endothelium be added to the BCOP assay and that optimization of the assay will require modification of the cornea holders.

Intermediate filament, laminin and integrin expression in lacrimal gland acinar cells: comparison of an immortalized cell line to primary cells, and their response to retinoic acid

PURPOSE

The goal of this study was to characterize intermediate filament, integrin and laminin expression by rabbit lacrimal gland acinar cells in culture, to determine whether retinoic acid (RA) alters expression of these proteins and to compare primary cells to an immortalized rabbit lacrimal gland acinar cell line using flow cytometric analysis.

METHODS

Primary cells, maintained in serum free medium, were exposed to 10(-6) M retinoic acid for 24 hours. Immortalized cells were grown in defined medium with Nu-Serum and exposed to retinoic acid. Cells were labeled with monoclonal antibodies to cytokeratins (AE1, AE2, AE3, AE5, CK10/13, CK18), integrins (alpha(3), alpha(6), alpha(V), beta(1), beta(2), beta(3) and beta(4)), laminin, or vimentin and with FITC-conjugated secondary antibodies. Cells were analyzed for antigen expression by flow cytometry and immunocytochemistry.

RESULTS

Primary and immortalized cells expressed type I and type II epithelial cytokeratins (AE1 and AE3), cytokeratin 18, and cytokeratin 3 (AE5) Both cell types were negative to AE2 and CK10/13. Primary and immortalized cells expressed vimentin in culture, with immortalized cells expressing this protein at higher levels. Lacrimal acinar cells appear to synthesize laminin which was detected intracellularly in both cells types. Integrins alpha(6) (CD49f) and alpha(V) (CD51) were expressed by primary and immortalized cells. Expression of integrin alpha(6) was 10-fold higher in immortalized cells compared to primary cells. Retinoic acid increased integrin alpha( V) expression by primary and immortalized cells 1.3-fold and 3-fold, respectively, and caused a slight increase in integrin alpha(6) expression by primary cells. Both cell types also expressed integrins beta( 1), beta(2) and beta(3), but beta(4) was detected only in immortalized cells. Lacrimal acinar cells do not express integrin alpha(3).

CONCLUSIONS

Expression of cytokeratins, laminin and integrins by primary and immortalized cells was similar, suggesting that the immortalized cell line is a good model for the study of lacrimal structure and function. Since retinoic acid up-regulated only integrin alpha(V), but not cytokeratins, these cells appear to be highly differentiated. Flow cytometry is a useful method for analysis of protein expression by lacrimal acinar cells.

N-linked glycoside and glucuronide conjugates of the retinoid, acitretin, are biologically active in cornea and conjunctiva

The purpose of this study was to test two water-soluble, synthetic retinoids, glucoseamido acitretin and glucuronamido acitretin, for biological activity in cells of the cornea and conjunctiva. Vitamin A-deficient, xerophthalmic rats were treated topically with these retinoids, and corneas were examined histologically for effects on epithelial keratinization. The effect of these retinoids on the proliferation of rabbit conjunctival fibroblasts in culture was also investigated. Glucoseamido acitretin treatment restored a normal cornea after eight to nine days of treatment, while no improvement was observed in the vehicle-treated corneas. Likewise, glucuronamido acitretin application restored a normal corneal surface and reversed keratinization after eight to ten days of treatment. These retinoids caused no irritation of the eye or ocular adnexa. In culture, exposure of conjunctival fibroblasts to glucoseamide acitretin inhibited cell proliferation. Cultures exposed to glucoseamido acitretin at 10(-8) M or 10(-6) M had cell densities 77.3% and 51.9% of control, respectively, after seven days. Glucuronamido acitretin also inhibited cell proliferation. Cultures exposed to glucuronamido acitretin at 10(-8) M had a cell density of 69.2% of control at day seven, while at 10(-6) M this retinoid completely inhibited cell proliferation. These results show that glucoseamide acitretin and glucuronamido acitretin are biologically active in the cornea and conjunctiva, and may be considered for ophthalmic use in diseases involving abnormalities of ocular surface cell differentiation or hyperproliferation of fibroblasts.

Expression of nuclear retinoic acid receptor and retinoid X receptor mRNA in the cornea and conjunctiva

PURPOSE

The effects of retinoic acid in cells are mediated by the nuclear retinoic acid receptors (RARs) and retinoid X receptors (RXRs). Although vitamin A is essential for the normal development and maintenance of the ocular surface, the RARs and RXRs have not been studied in cornea and conjunctiva. The purpose of this study was to identify the mRNA for these receptors in corneal and conjunctival cells in culture and to determine whether all-trans retinoic acid is able to induce expression of RAR mRNA.

METHODS

Total RNA was extracted from cultured rabbit corneal stroma and conjunctival fibroblasts and rabbit corneal epithelial cells. RNA was analyzed by Northern blotting using the cDNA probes for RAR alpha, RAR beta, RAR gamma, RXR alpha, RXR beta and RXR gamma mRNA. To investigate induction of retinoid receptors, cells were exposed to 10(-6) M all-trans retinoic acid for 2-48 h before preparation of RNA. Effects of retinoic acid on cell proliferation were also investigated.

RESULTS

RAR alpha mRNA transcripts (3.7 kb), RAR beta mRNA transcripts (3.3 kb) and RAR gamma mRNA transcripts (3.3 kb) are expressed by all the cell types studied, as are the RXR alpha mRNA transcripts (5.0 kb) and RXR beta mRNA transcripts (3.3 kb). RXR gamma mRNA is not detectable in corneal and conjunctival cells. All-trans retinoic acid induced RAR beta mRNA expression in corneal and conjunctival fibroblasts. Increased mRNA levels were detectable after 4-8 h and peaked by 24 h. RAR beta mRNA was not induced by retinoic acid in corneal epithelial cells. Retinoic acid also inhibited proliferation of conjunctival and corneal fibroblasts but had no effect on growth of corneal epithelial cells.

CONCLUSIONS

The expression of RARs and RXRs in the cornea and conjunctiva is similar to that reported in other tissues. The identification of these receptors may lead to a better understanding of gene transcription pathways in the cornea and conjunctiva and of the mechanisms that control keratinization, differentiation and proliferation of the cells of these tissues. The data suggest a relationship between the induction of RAR beta mRNA expression and inhibition of cell proliferation by retinoic acid.

The lacrimal gland expresses nuclear retinoid X receptors

PURPOSE

The lacrimal gland expresses nuclear retinoic acid receptors. This suggests that retinoids are involved in control of gene expression in the lacrimal gland. Retinoid X receptors (RXRs) form heterodimers with and are required for activation of retinoic acid receptors. The purpose of this study was to identify retinoid X receptors in the lacrimal gland.

METHODS

Total RNA was purified from rat, rabbit and human lacrimal glands and from cultured rat lacrimal cells. RNA was analyzed by northern blotting using cDNA probes for RXR alpha, RXR beta and RXR gamma. Nuclear protein extracts from rat and rabbit lacrimal glands were probed for RXR beta by immunoblotting, using a mouse monoclonal antibody.

RESULTS

RXR alpha mRNA transcripts (5 kb) and RXR beta mRNA transcripts (3.3 kb) are present in the lacrimal glands of all species studied and in cultured rat lacrimal cells. RXR gamma mRNA (1.9 kb) was detected only in the rabbit lacrimal gland. RXR beta is expressed as a 50 kDa protein in rat and rabbit lacrimal glands.

CONCLUSIONS

This study confirms the presence of RXRs in the lacrimal gland, thereby supporting a role for retinoids and their nuclear receptors in the control of gene transcription in the lacrimal gland.

Biological activity of N-(4-hydroxyphenyl) retinamide-O-glucuronide in corneal and conjunctival cells of rabbits and humans

Previous studies of topical retinoic acid for treatment of ocular surface disease met with limited success due to instability and irritancy of the retinoid and lack of efficacy in keratoconjunctivitis sicca. There has, however, been continued interest in the treatment of mucin deficiency and cicatrizing conjunctival diseases, such as ocular cicatricial pemphigoid (OCP), topically with retinoids. In this study the biological activity of stable, water-soluble, synthetic retinoid, N-(4-hydroxyphenyl) retinamide-O-glucuronide (4-HPROG) was investigated in vivo and in vitro using conjunctival and corneal epithelium and fibroblasts. Vitamin A-deficient rabbits with stage 3-4 corneal xerosis and squamous metaplasia confirmed by conjunctival impression cytology were treated with topical 0.1% 4-HPROG in an artificial tear vehicle for 3 weeks. Impression cytology was repeated at 2 and 3 weeks and at 3 weeks conjunctival biopsies were fixed for histology. Growth curves were generated using conjunctival fibroblasts of rabbits and humans (normals and patients with cicatrizing conjunctival disease including OCP and Stevens-Johnson syndrome) cultured in the 10(-8)-10(-6) M 4-HPROG. In vivo, corneal xerosis cleared in three days. A normal conjunctival epithelium was restored by 2 weeks and goblet cells were present by 3 wk, with no change in vehicle-treated controls. No ocular irritation occurred. In vitro, 10(-6) M 4-HPROG inhibits growth of rabbit conjunctival fibroblasts. The retinoid had no effect on proliferation of conjunctival fibroblasts from normal humans but the doubling time of cells from patients with OCP increased significantly, from 50.9 +/- 10.01 h (control) to 61.5 +/- 8.95 h (retinoid). Proliferation of conjunctival fibroblasts from a patient with Stevens-Johnson syndrome was also inhibited. N-(4-hydroxyphenyl) retinamide-O-glucuronide is biologically active and merits further study to determine its efficacy in controlling conjunctival fibrosis and treating ocular surface squamous metaplasia.

Nuclear retinoic acid receptors in the lacrimal gland

The lacrimal gland secretes and metabolizes retinoids and responds to retinoic acid in culture. Like other retinoid responsive organs it is expected to express the nuclear retinoid receptors. The goal of this study was to identify the retinoic acid receptors (RAR) in the lacrimal glands of rats, rabbits, and humans. Total RNA was prepared from whole lacrimal glands and rat lacrimal gland acinar cells grown in culture. RNA was subjected to Northern blot analysis and probed for the RAR alpha, RAR beta, and RAR gamma mRNAs. Nuclear extracts of rat and rabbit lacrimal glands were incubated with 3H-all-trans retinoic acid and analyzed by gel filtration chromatography. Western blots of the nuclear extracts were probed using monoclonal antibodies to RAR alpha and RAR beta. Rat lacrimal gland expresses RAR alpha mRNA with two transcripts (3.8 and 3.0 kb), a single RAR beta mRNA transcript (3.3 kb), and a single RAR gamma mRNA transcript (3.3 kb). Cultured rat lacrimal acinar cells also expressed the mRNA for all three RAR subtypes. Rabbit lacrimal glands express mRNAs for RAR alpha (3.7 and 2.9 kb) and RAR beta (3.2 kb) but RAR gamma mRNA is not detectable. Human lacrimal glands also express mRNA for RAR alpha (3.5 and 2.3 kb), RAR beta (3.4 kb) and RAR gamma (3.0 kb). Lacrimal gland nuclear extracts contain proteins in the 50 kDa range that specifically bind retinoic acid with Kd = 1.25 nM in rat lacrimal gland and 0.3 nM in rabbit. The monoclonal antibodies identified RAR alpha and RAR beta in both rat and rabbit lacrimal glands. The results of this study support a role for retinoids in maintaining the structure and function of the lacrimal gland. The presence of RARs suggests potential interactions of these receptors with other members of their superfamily, including androgen and thyroid receptors, which also may be involved in lacrimal function.

Effects of preservative-free artificial tear solutions on corneal epithelial structure and function

OBJECTIVES

To test the efficacy of a bicarbonate-containing artificial physiologic tear solution (solution PT) in providing an environment in which the damaged corneal epithelium can recover its normal barrier function and to compare this solution with other available artificial tears. Also, to investigate the effects on the corneal mucin layer and epithelial ultrastructure.

METHODS

The corneal epithelial permeability of anesthetized rabbits was increased by exposure to 0.1% benzalkonium chloride. The corneas were then exposed to solution PT, with or without bicarbonate, or one of four commercially available artificial tear solutions for 1.5 hours, followed by a 5-minute exposure to 5(6)-carboxyfluorescein. Frozen sections of the corneas were examined by fluorescence microscopy. The fluorescence intensity (FI) of the epithelium was measured by image analysis. Undamaged corneas exposed to tear solutions were examined by transmission electron microscopy after fixation of the mucin layer with cetylpyridinium chloride.

RESULTS

The FI of corneas damaged by benzalkonium chloride was increased threefold above those of undamaged controls. Damaged corneas treated with either of two commercial isotonic tear solutions partially recovered their barrier function, but the FI did not reach control levels. Corneas treated with hypotonic solutions containing ethylenediaminetetraacetic acid (EDTA) did not recover. In contrast, the FI of corneas treated with solution PT returned to control levels. This effect was lost in the absence of bicarbonate. Solution PT and the two isotonic solutions maintained normal corneal ultrastructure and mucin layer. Lack of bicarbonate in solution PT resulted in focal damage to superficial epithelial cells, whereas the EDTA-containing solutions destroyed the first two cell layers and reduced the mucin thickness.

CONCLUSIONS

Bicarbonate-containing solution PT is superior to the other tear solutions tested in promoting recovery of the damaged corneal epithelial barrier and maintaining normal ultrastructure. The presence of bicarbonate appears to be essential to this process.

The influence of retinoic acid on growth and morphology of rat exorbital lacrimal gland acinar cells in culture

The lacrimal gland transports and metabolizes retinoids and may require vitamin A for normal function. To study effects of retinoic acid on morphology and growth of the lacrimal gland, rat lacrimal acinar cells were cultured in medium with serum or in serum-free medium in the presence or absence of retinoic acid. In the presence of serum, the acinar cells have a somewhat fibroblastic morphology and form confluent layers. Addition of retinoic acid to these cultures causes formation of tubule-like structures. Retinoic acid inhibits the growth of lacrimal cells in medium with serum and the cells do not reach confluence; however, the labeling of the cells with bromodeoxyuridine is not affected by retinoic acid. In serum-free medium the growth of acinar cells is reduced, but their morphology is epithelial and structures resembling secretory domes are present. Retinoic acid causes a further reduction in growth, domes are absent, and cell spreading and enlargement occurs. The effects of retinoic acid on growth and morphology of lacrimal acinar cells in culture are complex and the relevance of these observations to lacrimal function in vivo is unclear; the study demonstrates, however, that these cells are responsive to retinoic acid.

The rabbit lacrimal gland in vitamin A deficiency

PURPOSE

To investigate the morphology, ultrastructure, and protein secretion of the vitamin A-deficient rabbit lacrimal gland.

METHODS

The lacrimal glands of vitamin A-deficient rabbits and age-matched controls were fixed, processed by standard methods, and examined by light and transmission electron microscopy. Protein secreted by the lacrimal gland was analyzed using gel filtration chromatography and electrophoresis.

RESULTS

By light microscopy, the glands of experimental and control rabbits were indistinguishable. Electron microscopy showed little effect of vitamin A deficiency on the lacrimal acini, although occasional pyknotic nuclei were observed. The intralobular ductal epithelium was unaffected. Protein concentration and composition were essentially unchanged in lacrimal gland fluid of vitamin A-deficient rabbits compared to controls.

CONCLUSIONS

The rabbit lacrimal gland is minimally affected by vitamin A deficiency, suggesting species differences between rabbits and rats in the vitamin A requirements of the lacrimal gland. Normal lacrimal gland structure and function in vitamin A deficiency allow for the prompt secretion of retinol on the restoration of vitamin A to the diet.

Cellular retinol-binding protein and cellular retinoic acid-binding protein in the lacrimal gland

Cells that require or metabolize vitamin A contain cellular retinol-binding proteins (CRBP) and cellular retinoic acid-binding proteins (CRABP) which apparently participate in metabolism and transport of retinoids within the cell. Since the lacrimal gland secretes and metabolizes vitamin A, the cellular retinoid-binding proteins and their mRNAs should also be present in this gland. Total RNA from rat and rabbit lacrimal glands was analysed by RNase protection using 32P-labeled antisense cRNA probes. CRBP-I mRNA is present in rat lacrimal gland at 2% of the level in liver but CRBP-I mRNA could not be identified conclusively in rabbit lacrimal gland using the available rat cRNA probe. CRABP-I mRNA is present in rat lacrimal gland at 5% of the level in the 12-day gestation rat fetus, but was not detectable in the rabbit lacrimal gland. Rabbit or rat lacrimal gland cytosol was incubated with [3H]retinol or [3H] retinoic acid followed by ion-exchange chromatography using a Tris buffer and NaCl gradient. CRBP is present in rabbit and rat lacrimal gland at 78.5 +/- 14.5 and 71.1 +/- 11 pmol g-1 protein, respectively. CRABP is present in rat lacrimal gland at 190.6 +/- 13.8 pmol g-1 protein but was not detectable in rabbit lacrimal gland. The presence of CRBP in the lacrimal gland is consistent with its role in secretion of retinol into the lacrimal gland fluid. The apparent lack of CRABP in rabbit lacrimal gland as compared with rat suggests species differences in the role of retinoids in differentiation and maintenance of this organ.

Artificial tear composition and promotion of recovery of the damaged corneal epithelium

In severe dry eye syndromes the corneal epithelium is compromised with development of punctate erosions and increased permeability. In the present study the ability of artificial tear solutions to promote recovery of the corneal epithelial barrier was determined by measurement of corneal uptake of 5,6 carboxyfluorescein (CF). Corneas of anesthetized rabbits were exposed to 0.01% benzalkonium for 5 min to increase epithelial permeability. The cornea was then exposed to an artificial tear solution for 1.5 h followed by measurement of CF uptake. During exposure to three commercial isotonic, nonpreserved solutions and a solution preserved with polyquaternium-1, CF uptake decreased significantly but did not return to control. No recovery of the epithelial barrier occurred during exposure of corneas to nonpreserved hypotonic solutions. During exposure to an experimental tear solution with an electrolyte composition similar to human tears, buffered with bicarbonate, CF uptake returned to control levels. Bicarbonate is an essential component of this solution because the same formula buffered with borate or without buffer was ineffective in promoting recovery of the damaged corneal epithelium.

Retinyl ester hydrolysis in the rabbit lacrimal gland

The lacrimal gland stores retinyl esters which are synthesized by the enzyme acyl CoA:retinyl acyl transferase. Retinol is released from retinyl ester reserves by retinyl ester hydrolase (REH). Since the lacrimal gland secretes retinol, this gland should also contain this enzyme. To identify bile salt-dependent REH activity, rabbit lacrimal glands were homogenized in 0.05 M Trismaleate buffer, and enzyme activity was determined in the tissue homogenate, in the membrane fraction and in the cytosolic fraction by measurement of production of retinol from retinyl palmitate (nmol retinol produced/mg protein/h). In the lacrimal gland, production of retinol was optimal in the presence of 200 mM CHAPS at pH 7. The REH activity in the presence of 1000 microM retinyl palmitate was 2.38 +/- 0.18 nmol/mg/h in the homogenate, 1.13 +/- 0.16/nmol/mg/h in membranes and 3.25 +/- 0.26 nmol/mg/h in cytosol. By comparison, REH activity in rabbit liver was 6.58 +/- 0.75 nmol/mg/h. The REH activity in lacrimal gland was not affected by vitamin A deficiency. These data are consistent with the presence of retinyl ester hydrolase activity in the lacrimal gland and provide further evidence that this gland is adapted for metabolism and secretion of retinol.

The lacrimal gland synthesizes retinol-binding protein

The rabbit lacrimal gland secretes retinol bound to a 20-21 kDa protein. To test the hypothesis that this protein might be retinol-binding protein (RBP) we probed lacrimal gland for RBP mRNA and lacrimal gland fluid for RBP. A rabbit RBP cDNA clone was used to probe rabbit and rat lacrimal gland RNA using RNase protection analysis. The lacrimal gland contains RBP mRNA at a level 0.1 to 0.03% of that observed in the liver. This RBP mRNA was identical to that observed in the liver based on RNase protection analysis, Northern blot analysis and primer extension analysis. The RBP mRNA levels in the lacrimal gland were not altered by the retinol status of the rabbits. We analysed lacrimal gland fluid for RBP by immunoblotting using a monoclonal antibody that recognizes rat, human and rabbit RBP. A single protein band from the rabbit lacrimal fluid bound the antibody, and this protein comigrated with human RBP which also bound the antibody. We conclude that the lacrimal gland contains RBP mRNA and that the lacrimal gland synthesizes and secretes RBP into the lacrimal gland fluid. This is the first demonstration that an extrahepatic tissue containing RBP mRNA synthesizes and secretes the protein in vivo.

Quantitative evaluation of the corneal epithelial barrier: effect of artificial tears and preservatives

The effect of artificial tear solutions on the corneal epithelial barrier was evaluated by measuring corneal uptake of 5, 6 carboxyfluorescein (CF) after exposure of rabbit corneas to various formulations in a conjunctival cup. Four tear solutions containing 0.01% benzalkonium chloride (BAC), a formulation containing 0.001% Polyquad, a contact lens re-wetting solution containing 0.004% thimerosal, and each of the above preservatives in balanced salt solution (BSS) were evaluated. Four non-preserved solutions were also tested. After treatment with the solutions corneas were exposed to the CF, removed from the eyes and dialyzed in balanced salt solution. The CF concentration in the dialysate was measured by fluorometry. Solutions containing 0.01% BAC caused a 9.24 to 99.28 fold increase in CF uptake as compared to control. Solutions preserved with Polyquad or thimerosal caused only a 0 to 4 fold increase in CF uptake while non-preserved solutions caused no change compared to control. Corneas prepared for transmission electron microscopy using fixative containing ruthenium red exhibited damage which correlated well with CF uptake; the ruthenium red penetrated the epithelium to the basal cell layer after corneal exposure to solutions containing BAC while only superficial cell layers were stained after exposure to the other test solutions. The method used in this study allows statistical comparison of artificial tear formulations. The data show that patients with severe dry-eye who use artificial tears frequently should avoid tear solutions containing BAC and that non-preserved solutions are preferable for treatment of these patients.

The effect of retinoic acid on thymidine incorporation and morphology of corneal stromal fibroblasts

The effect of retinoic acid on DNA synthesis and cell morphology was studied using corneal stromal fibroblasts in culture. All-trans retinoic acid induces an increase in DNA synthesis after 24 hours of exposure. Autoradiographic studies of 3H-thymidine incorporation into corneal stromal cells exposed to 10(-6) M retinoic acid for 24 hours showed an increase in labeling which ranged from 19.2% to 67.6% over control cultures. Scintillation analysis of labeled cultures also showed an increase in incorporation of 3H-thymidine into cells treated with 10(-6) M retinoic acid, with increases ranging from 21.8% to 114.7% above control cultures. Exposure of cultured corneal stromal cells to 10(-6) M retinoic acid resulted in a dramatic change in cell morphology such that they changed from spindle-shaped to round, flattened cells which were epithelioid in appearance. These data demonstrate that biological activity of retinoic acid in stromal fibroblasts and imply a role for vitamin A in maintenance of stroma structure and function.

Esterification of retinol in lacrimal gland. Evidence for acyl-CoA:retinol acyltransferase activity

Vitamin A is stored in cells as long-chain fatty acyl esters of retinol. Esterification in many tissues is catalyzed in part by acyl-CoA:retinol acyltransferase (ARAT). Since the lacrimal gland contains stores of retinyl esters, it was the goal of this study to determine whether the lacrimal gland contains ARAT activity. Rabbit lacrimal gland microsomes incubated with 3H-retinol synthesized retinyl esters. The reaction rate was stimulated 30-fold in the presence of a fatty acyl-CoA generating system, producing a mixture of esters including retinyl laurate, retinyl linoleate, retinyl palmitate, and retinyl stearate as determined by reverse-phase HPLC. Retinyl palmitate was synthesized at 1944 pmole/mg protein/30 min, representing 50% of total ester synthesis, and this activity was directly proportional to microsomal protein concentration. In the presence of 180 microM 3H-retinol and 100 microM palmitoyl-CoA, retinyl palmitate was synthesized at 175-220 pmole/mg/min, and the reaction fit Michaelis-Menten kinetics as a function of retinal concentration (theoretical Vmax = 329.4 pmole/mg/min). Lauroyl CoA and stearoyl CoA, but not linoleoyl CoA, were as effective as palmitoyl CoA as substrates for the reaction. The enzyme activity was inhibited by p-chloromercuriphenyl sulfonic acid and Na-taurocholate. The data show that the lacrimal gland synthesizes retinyl esters and that the characteristics of synthesis are consistent with the presence of acyl-CoA:retinol acyltransferase in lacrimal gland.

13-cis retinoyl-beta-glucuronide in lacrimal gland fluid of rabbits treated with 13-cis retinoic acid

When rabbits and humans are treated with orally administered 13-cis retinoic acid, the retinoic acid and a more polar retinoid metabolite appear in tears and lacrimal gland fluid. This study tested the hypothesis that this metabolite was a retinoyl-beta-glucuronide. Lacrimal gland fluid from rabbits treated with a pharmacologic dose of 13-cis retinoic acid was analyzed. Based on chromatographic retention time, absorbance maximum, and degradation to 13-cis retinoic acid by the enzyme beta-glucuronidase, it was shown that this retinoid metabolite is 13-cis retinoyl-beta-glucuronide. The 13-cis retinoyl-beta-glucuronide was also extracted from the lacrimal gland itself. It is concluded that 13-cis retinoyl-beta-glucuronide is a major metabolite of 13-cis retinoic acid in the rabbit and that this retinoid is secreted by the lacrimal gland.

Electrolyte composition of lacrimal gland fluid and tears of normal and vitamin A-deficient rabbits

Rabbit tears and lacrimal gland fluid were collected simultaneously during pilocarpine stimulation with the goal of comparing the ionic composition of these fluids at various flow rates. Ions measured were sodium, potassium, calcium, magnesium, zinc, chloride, and bicarbonate. Human tears were also analyzed for purposes of comparison. Generally, tears and lacrimal gland fluid do not differ in ionic composition except for zinc and bicarbonate, which are in higher concentration in tears than in lacrimal gland fluid. The ionic composition of tears and lacrimal gland fluid of vitamin A-deficient rabbits was also analyzed. The maximal flow rate of lacrimal gland fluid was decreased in vitamin A-deficient rabbits as were calcium levels in tears and lacrimal gland fluid, as compared with controls. Concentrations of other ions generally did not differ from normal levels, indicating that vitamin A deficiency has only moderate effects on lacrimal gland function in the rabbit.

The relationship between secretion of retinol and protein by the lacrimal gland

Secretion of retinol and protein by the rabbit lacrimal gland appear to be closely related, suggesting that they are secreted by the same mechanism. Pilocarpine and vasoactive intestinal peptide (VIP) both stimulate protein and retinol secretion rate in a dose-dependent manner, but the concentrations of retinol and protein in the lacrimal gland fluid are independent of fluid flow at flow rates in excess of 1 microliter/min. Under all conditions of stimulation with pilocarpine and VIP that were studied, the retinol:protein ratio in lacrimal gland fluid of normal rabbits remained constant at 3.3 ng retinol/mg protein. This correlation between retinol and protein secretion by the lacrimal gland suggests that retinol is protein-bound in lacrimal gland fluid. To identify this protein, vitamin A-deficient rabbits were treated orally with 3H-retinyl acetate. Lacrimal gland fluid was collected and analyzed for 3H-retinol and protein. The 3H-retinol in lacrimal gland fluid was identified by reverse-phase HPLC and analysis of protein by gel filtration chromatography showed that this 3H-retinol was associated with protein which eluted from the columns in the 20 kD range.

Drug uptake and release by a hydrogel intraocular lens and the human crystalline lens

Implantation of a hydrogel (IOGEL) intraocular lens in humans has been reported. The polyhydroxyethyl methacrylate (poly HEMA) matrix of this hydrogel is permeable to water soluble drugs and may adsorb agents used intracamerally during cataract extraction or topically during the postoperative period. This study compared the in vitro uptake and release of chloramphenicol, dexamethasone, epinephrine, pilocarpine, and bovine serum albumin by polymethylmethacrylate and hydrogel intraocular lenses with that of the intact crystalline lens of humans and rabbits. An in vivo study compared the uptake and release of chloramphenicol and dexamethasone by hydrogel lenses implanted in the anterior chamber of rabbit eyes with that of the rabbit's crystalline lens. The in vitro uptake and washout of epinephrine and pilocarpine by the hydrogel lens was comparable to the human lens. Uptake of chloramphenicol and dexamethasone by the hydrogel lens exceeded that of the human lens and, following a two-hour washout period, the dexamethasone content of the hydrogel lens remained significantly greater than the human lens. The uptake and washout of bovine serum albumin by the hydrogel lens was half that of the human lens. In vivo, the hydrogel lens efficiently eluted both chloramphenicol and dexamethasone. These studies show that a hydrogel lens will not act as a significant depot for drugs in the eye.

Vitamin A is stored as fatty acyl esters of retinol in the lacrimal gland

Many tissues which require vitamin A store the vitamin as long-chain fatty acyl esters of retinol. As part of a study designed to characterize vitamin A metabolism in the lacrimal gland, which transports retinol from blood to lacrimal gland fluid, extracts from lacrimal glands of rabbits and rats were analyzed by non-aqueous high performance liquid chromatography. Retinyl linoleate, retinyl palmitate, and retinyl stearate were identified in these extracts by their co-elution with standards, their retention time relative to retinyl palmitate, and their susceptibility to hydrolysis by saponification. Retinyl palmitate was present in rabbit lacrimal gland at 51.0 +/- 10.1 ng/g tissue. After treatment of vitamin A-deficient rabbits with orally administered [11,12-3H] retinyl acetate, the radiolabeled esters retinyl linoleate, palmitate, and stearate were extracted from the lacrimal glands. These data show that the lacrimal gland stores vitamin A as fatty acyl esters of retinol.

Tear secretion and lacrimal gland function of rabbits treated with isotretinoin

Adverse ocular reactions including dry eye symptoms and blepharoconjunctivitis are common side effects of treatment with isotretinoin (13-cis-retinoic acid). However, there is little agreement in the literature on the effect of this drug on the tears. Because we have previously shown that the lacrimal gland secretes isotretinoin, we conducted a study of the effect of isotretinoin on lacrimal gland function. Rabbits were treated with isotretinoin for 5 months. Throughout the study tear secretion was monitored by the Schirmer test. At the end of the study lacrimal gland function was assessed by measurement of fluid and protein secretion rates and secretion of retinol in response to a pilocarpine stimulus. Lacrimal gland function was not affected by isotretinoin as compared with a group of age-matched control rabbits; however, Schirmer test scores were significantly increased in the treated animals as compared with control values. We conclude that isotretinoin is not toxic to the lacrimal gland of rabbits. This suggests that ocular irritation in patients treated with isotretinoin is not caused by decreased tear secretion during therapy.

Treatment of corneal xerophthalmia in rabbits with micromolar doses of topical retinoic acid

Several reports have appeared on the efficacy of topically applied 0.01% or 0.1% all-trans retinoic acid (0.04-0.4 millimolar) for treatment of xerophthalmia, conjunctival squamous metaplasia, and corneal epithelial erosions in humans and animals. An observation common to many of these studies is the occurrence of an adverse reaction to retinoic acid in the form of lid margin hyperemia and blepharoconjunctivitis. Since retinoic acid is biologically active at micromolar to nanomolar concentrations, it may be possible to reduce side effects while maintaining therapeutic effectiveness by reducing the retinoic acid concentration in ophthalmic formulations. In the present study, topical 0.005% retinoic acid in petrolatum ointment reversed corneal keratinization in xerophthalmic, vitamin A-deficient rabbits in 3-4 days while 0.0005% (2 micromolar) retinoic acid ointment was effective in 4-6 days. Further clinical trials of topical retinoic acid for treatment of ocular surface disease should be conducted using micromolar concentrations of retinoic acid which are expected to maintain a therapeutic effect while reducing adverse reactions.

Isotretinoin in lacrimal gland fluid and tears

Isotretinoin (13-cis-retinoic acid) is used in the treatment of severe cystic acne. Adverse ocular reactions, including blepharoconjunctivitis and dry eye symptoms, are frequent side effects of this drug. Our previous observation that retinol is present in tears and lacrimal gland fluid suggests that isotretinoin may also be secreted by the lacrimal gland. Rabbits were treated with isotretinoin, and lacrimal gland fluid was collected from the cannulated lacrimal gland duct. Tears were collected from patients who were being treated with isotretinoin. Lacrimal gland fluid and tears were analyzed by reverse-phase high-pressure liquid chromatography and a peak eluted from each sample, which was identified as isotretinoin. We conclude that the lacrimal gland is able to secrete isotretinoin in addition to retinol and that, in animals and patients treated systemically with isotretinoin, the ocular surface is exposed to the drug via the tear film.

Retinol secretion by the lacrimal gland

In order to determine the source of the retinol which has been identified in the tear fluid, the lacrimal gland ducts of rabbits and rats were cannulated and the collected lacrimal gland fluid was analyzed by high performance liquid chromatography. Retinol was identified in the lacrimal gland fluid of rabbits and rats, and it is concluded that the lacrimal gland is the source of retinol in the tears. Dose-response studies show that intravenously administered pilocarpine and intra-arterial acetylcholine stimulate secretion of retinol by the lacrimal gland. Intravenous administration of vasoactive intestinal peptide (VIP) also stimulates retinol secretion in a dose-response manner. These observations are similar to the effects of cholinergic drugs and VIP on protein secretion by the lacrimal gland.

Corneal endothelial healing rate and the effect of topical retinoic acid

These studies were undertaken to evaluate wound healing rates of the corneal endothelium in vivo. After insertion of a 26-gauge needle into the anterior chamber of the rabbit eye through the limbus, a 5-0 nylon monofilament was introduced through the needle, and endothelial wounds were made by scratching the cells with the filament. The wounds were photographed with a wide-field specular microscope at various intervals. Montages of the wounds were made, and the areas of the wounds were determined by planimetry. Wound closure occurred rapidly in a linear manner during the first 6 hr after wounding, after which the rate of cell migration decreased. Healing rates (micron2/hr) during the first 6 hr were calculated by linear regression analysis. There was a direct linear correlation between the healing rate and initial wound area. The slope of this line for nine normal (untreated) corneas was 0.093 hr-1. Nine corneas were treated with 0.1% retinoic acid in petrolatum ointment, while eight control corneas received vehicle alone. The slope of healing rate versus initial wound area for treated corneas (0.11 hr-1) was significantly greater than control (0.097 hr-1). This was interpreted as a stimulation of corneal endothelial migration during healing by retinoic acid. As a result of this study, a method for analysis of corneal endothelial healing rate has been developed which can be used for comparison of healing rates among treatments when initial wound area cannot be standardized.

The efficacy of retinoic acid ointment for treatment of xerophthalmia and corneal epithelial wounds

In previous studies of topical application of retinoic acid to the eye, retinoic acid in an oil vehicle has been used. An all-trans-retinoic acid (Tretinoin, USP) formulation in a petrolatum-based ophthalmic ointment vehicle has now been developed which is chemically stable for at least one year. The ointment is effective at retinoic acid concentrations of 0.01-0.1% in reversing corneal keratinization in vitamin A-deficient, xerophthalmic rabbits. At 0.1% it also stimulates the healing rate of corneal epithelial wounds in rabbits and monkeys. Retinoic acid ointment may prove to be valuable clinically in the treatment of xerophthalmia, corneal epithelial erosions, and related ocular surface abnormalities.

In vivo metabolism of topically applied retinol and all-trans retinoic acid by the rabbit cornea

Corneas of normal and vitamin A-deficient rabbits were treated topically with [11, 12-3H] retinol or [11, 12-3H] all-trans retinoic acid. Methanol extracts of these corneas were analyzed by high pressure liquid chromatography. Radiolabeled compounds were extracted from the corneas which co-migrated chromatographically with known retinoid standards. In agreement with studies on other tissues and organs, retinol was metabolized to retinoic acid and more polar compounds by corneas of normal and vitamin A-deficient rabbits. All-trans retinoic acid was isomerized to 13-cis retinoic acid in normal rabbit corneas; however, this trans-cis isomerization did not occur in vitamin A-deficient, xerophthalmic corneas.

A larger corneal epithelial wound closes at a faster rate

In order to evaluate relationships between healing rates and initial wound area, epithelial wounds were made on rabbit corneas by scraping the epithelium within a 4-, 6.5-, or 8-mm trephine mark. The wounds were stained with fluorescein and photographed during healing. The wounded areas were measured by planimetry. Although larger wounds closed later than smaller wounds, all of the healing curves appeared to be linear. The mean healing rate of the 8-mm diameter wounds (0.91 mm2/hr) was significantly greater than that of the 6.5-mm diameter wounds (0.80 mm2/hr). The 4-mm diameter wounds healed at a significantly slower rate (0.37 mm2/hr) when compared to the 6.5-mm diameter wounds. The authors found a strong positive correlation between the healing rates and the initial wound areas. By comparison, regardless of the initial wound area, the wound diameter decreased at a rate of approximately 0.1 mm/hr, which may explain the dependency of the healing rate on the initial wound area. The healing rate varied considerably between animals with the same diameter wounds, but both eyes of each animal showed a similar healing rate.

Corneal epithelial wound healing in normal and diabetic rabbits treated with tretinoin

The rate of corneal epithelial wound healing has been shown to be faster in diabetic than in normal rabbits when the epithelial cells are removed by scraping or freezing; both methods of epithelial removal, however, damage the basement membrane in corneas of diabetic but not normal rabbits. In this study, we compared the rate of wound healing and the increase in corneal thickness in normal and diabetic rabbits in which the epithelial cells were removed with heptanol, a method that does not damage the basement membrane in either group. In addition, the effect of tretinoin on the rate of wound healing was compared in both groups. There was no statistical difference between the rate of epithelial healing in the untreated control and in untreated diabetic eyes. Treatment with tretinoin resulted in a significant increase in the rate of healing in control but not in diabetic eyes. Corneal thickness increased in all groups after epithelial removal, but the increase was significantly less in the corneas of diabetic rabbits at 24 hours. These results indicate that tretinoin may be more effective in promoting epithelial healing in eyes of normal patients than in diabetic patients.

Vitamin A is present as retinol in the tears of humans and rabbits

Vitamin A is required for the normal growth maintenance and maturation of the corneal epithelium and is effective in the treatment of xerophthalmia and experimental corneal epithelial wounds when applied topically as retinoic acid. The normal route of delivery of vitamin A to the cornea has remained undefined. We collected tears from normal and vitamin A deficient rabbits and from humans and analyzed them by high pressure liquid chromatography. A peak corresponding to a retinol standard was eluted from normal rabbit and human tears but was absent from the vitamin A deficient rabbit tears. The retinol concentration in rabbit tears was 69 ng/ml (0.2 X 10(-6)M) and in human tears was 16 ng/ml (0.5 X 10(-7)M). This demonstration that vitamin A is present in the tears as retinol establishes the rationale for treatment of corneal disease with topical vitamin A.

Choroidal rete mirabile function and resistance to retinal oxygen toxicity in fish

Previous studies have suggested that resistance to O2 toxicity in teleost fish retina is directly related to choroidal rete mirabile function and to the chronically elevated PO2 at the retina. We tested this hypothesis in several species of fish with varying degrees of rete development. The effect of hyperbaric oxygen on O2 consumption, Na+-, K+-ATPase activity nd the ERG was recorded. Hyperbaric oxygen had no effect on the ERG of either the flounder, which has a large rete and elevated PO2, or the eel, which has no rete. Exposure to hyperbaric oxygen for 6 hr attenuated by 50% the ERG of the goosefish which has a small rete (retinal PO2 = 90 mmHg). Hyperbaric oxygen abolished the b-wave of the dogfish ERG and attenuated the skate b-wave by 80%. Elasmobranchs have no rete and low retinal PO2. Hyperbaric oxygen had no effect on O2 consumption and ATPase activity in any species studied. The ERG data supported the hypothesis that resistance to oxygen toxicity by the teleost retina is an adaptation to a chronically elevated PO2 generated by the choroidal rete.

Healing of experimental corneal wounds treated with topically applied retinoids

We treated experimental corneal epithelial wounds in rabbits with topical retinoids. Treatment with 0.1% all-trans-retinoic acid three times per day resulted in a 21% increase in the healing rate compared to the control eyes. Treatment five times a day resulted in a 35% increase in healing rate. Treatment with topical retinoic acid also promoted corneal deturgescence. Retinyl palmitate, retinyl acetate, retinol, and 13-cis-retinoic acid had no effect on corneal wound healing. These data suggested that topically applied all-trans-retinoic acid may be effective in promoting corneal healing after surgery and in the treatment of persistent and recurring corneal epithelial defects.

Retinoid permeability and uptake in corneas of normal and vitamin A-deficient rabbits

In vitro perfusion of corneas of normal and vitamin A-deficient rabbits provided a model in which to study the pharmacokinetics of corneal permeability and uptake of retinoic acid and retinol. The permeability coefficients of retinoic acid and retinol were 1.49 x 10(-5) and 0.61 x 10(-5) cm/s, respectively. Removal of the corneal epithelium did not affect the permeability of these lipid-soluble retinoids; however, diffusion through xerophthalmic, vitamin A-deficient corneas was significantly reduced. The corneal uptake of retinoic acid and retinol was reduced by 50% on removal of the epithelium, was nonspecific, and was not affected by xerophthalmia. High-performance liquid chromatography indicated that these retinoids were not metabolized during diffusion through the cornea. These results show that topical application of retinoids is a rational approach to the treatment of such corneal diseases as xerophthalmia and epithelial defects.

A comparison of healing of corneal epithelial wounds stained with fluorescein or Richardson's stain

The effects of fluorescein and Richardson's stain on corneal epithelial wound healing were compared in eyes of rabbits whose corneas had the epithelium removed by scraping or by n-heptanol. One eye of each rabbit was stained with fluorescein and the other eye was stained with Richardson's stain at intervals throughout the healing process, and the wounds were photographed for planimetry and determination of re-epithelialization rate. Corneal thickness was also measured throughout the re-epithelialization. These studies showed that Richardson's stain, as compared with fluorescein, decreases re-epithelialization rate, delays wound closure, and slows the return of the edematous cornea to normal thickness. Therefore fluorescein rather than Richardson's stain should be used to stain epithelial defects in corneal wound healing studies and in the evaluation of the corneal toxicity of chemical agents.

Healing of corneal epithelial wounds in marine and freshwater fish

The corneal epithelium of a fish is in direct contact with the aquatic environment and is a barrier to movement of ions and water into and through the cornea. This tissue layer is thus important in maintenance of corneal transparency. When the epithelium is wounded, its protective function is lost and corneal transparency remains compromised until the epithelial barrier is re-established. This study was undertaken to investigate the healing response of the fish cornea to epithelial abrasion. Wounds were stained with fluorescein and photographed during healing. Wound areas were measured by planimetry. The cornea of the sculpin, a marine teleost, becomes edematous after wounding and heals at 2.54 to 3.42 mm2/hr. Nonswelling corneas of the elasmobranchs--dogfish shark and skate--heal at 1.29 mm2/hr, respectively. The wounded eye of the rainbow trout, a freshwater teleost, is stressed by the low osmolality of the environment. Severe corneal edema and cataracts develop following epithelial wounding, and the cornea heals at 0.64 mm2/hr. Although the healing rates in teleosts differ from those in mammals, histology shows that the corneal healing mechanism is essentially the same in fish and mammals.

Retinomotor activity and the c-wave of the hypoxic trout retina

The teleost retina exhibits retinomotor activity in response to changing light intensity. We have shown that hypoxia interfers with normal retinomotor activity in the dark-adapted rainbow trout, so that the retina assumes an essentially light-adapted configuration with the cones contracted, the rods extended toward the pigmented epithelium, and epithelial pigment expanded. These cell movements appear to be correlated with the marked increase in ERG c-wave amplitude which we consistently observe during hypoxia in trout. Since the a-wave is not immediately affected by hypoxia, this increase in c-wave amplitude may be related to the movement of the rods toward the pigmented epithelium, which would cause a greater than normal change in extracellular [K+] near the apical membrane in response to a light stimulus, leading to an increase in c-wave amplitude.